1. Field of the Invention
Aspects of the present invention relate to a methanation catalyst, and a carbon monoxide removing system, a fuel processor, and a fuel cell including the same, and more particularly, to a methanation catalyst having high selectivity for a methanation of carbon monoxide rather than a reverse water gas shift reaction of carbon dioxide, which is a side reaction of the methanation, maintains high concentration of generated hydrogen, and effectively remove carbon monoxide at a low operating temperature of 200° C. or less, and a carbon monoxide removing system, a fuel processor, and a fuel cell including the same.
2. Description of the Related Art
Fuel cells are electricity generation systems that directly convert the chemical energy of hydrogen from hydrocarbons, such as methanol, ethanol and natural gas, and the chemical energy of oxygen to electrical energy.
FIG. 1 is a schematic diagram illustrating an energy conversion process to produce electricity in a fuel cell 4. As illustrated in FIG. 1, air that includes oxygen is supplied to a cathode 1 and a fuel containing hydrogen is supplied to an anode 3, electricity is generated as an electrolyte membrane 2 allows hydrogen ions to flow from the anode 3 to the cathode 1 through the electrolyte membrane 2 while electrons e are forced to flow through a circuit, which produces usable energy. The hydrogen and oxygen combine at the cathode 1 to produce water.
Fuel cell systems consist of a fuel cell stack, a fuel processor (FP), a fuel tank, a fuel pump, etc. The fuel cell stack is a main body of a fuel cell, and includes several to several tens of unit cells each including a membrane electrode assembly (MEA) and a separator (or bipolar plate). With reference to FIG. 2, hydrogen is extracted from the fuel in a fuel processor 10 so as to be supplied to the fuel cell stack 20. The fuel processor 10 includes a desulfurizer 11, a reformer 12, a burner 13, a water supply pump 16, first and second heat exchangers 14a and 14b, and a CO removal unit 15 comprising a CO shift reaction device 15a and a CO removal device 15b. The hydrogen is extracted in a reformer 12 of the fuel processor 10.
The fuel pump supplies the fuel in the fuel tank to the fuel processor. The fuel processor 10 produces hydrogen by reforming and purifying the fuel and supplies the hydrogen to the fuel cell stack 20. The fuel cell stack 20 receives the hydrogen and generates electrical energy through electrochemical reactions of the hydrogen with oxygen.
The reformer 12 of the fuel processor 10 reforms a hydrocarbon fuel using a reforming catalyst. The hydrocarbon fuel contains a sulfur compound, which easily poisons the reforming catalyst. As such, it is necessary to remove the sulfur compound prior to reforming the hydrocarbon fuel. Thus, the hydrocarbon fuel needs to be desulfurized prior to the reforming process.
In addition to hydrogen, hydrocarbon reforming produces carbon dioxide and a small amount of carbon monoxide as by-products. Since CO poisons catalyst materials in electrodes of the fuel cell stack, reformed fuel cannot be directly supplied to the fuel cell stack. Thus, a CO removal process is needed. It is preferable to reduce the CO levels to less than 10 ppm.
CO can be removed using a high and low temperature shift reaction represented by Reaction Scheme 1 below.

A high-temperature shift reaction is performed at a temperature of 400 to 500° C. Generally, a high-temperature shift reaction is followed by a low-temperature shift reaction at a temperature of 200 to 300° C. Even though these reactions are performed, it is very difficult to reduce the CO levels to less than 5,000 ppm.
To solve this problem, a preferential oxidation (PROX) reaction represented by Reaction Scheme 2 below can be used.

Air is additionally supplied since the PROX reaction needs an excessive amount of oxygen. However, the concentration of hydrogen to be supplied to the fuel cell is decreased by the PROX reaction as carbon dioxide is generated.
Thus, a methanation reaction as shown in Reaction Scheme 3 is used to remove carbon monoxide and to overcome disadvantages of the PROX reaction.

Platinum, which is a precious metal, has been widely used in the PROX reaction, and ruthenium, which is also a precious metal, and cobalt and nickel loaded on an alumina support, which are non-precious metals, have been widely used in the methanation as catalysts.
The conventional methanation catalyst has high activity in the methanation of carbon monoxide as well as the methanation of carbon dioxide and the reverse water gas shift reaction, which are side reactions of the methanation of carbon monoxide and are shown below.


The side reactions consume hydrogen supplied to the fuel cell stack. Accordingly, it is necessary to develop a cost effective methanation catalyst that is simple to manufacture. Further, the methanation catalyst should be formed of a non-precious metal catalyst that does not require a support and that maintains high concentration of generated hydrogen as only small amounts of hydrogen and carbon dioxide are consumed in the reactions and has a high selectivity for the methanation of carbon monoxide rather than the methanation of carbon dioxide and the reverse water gas shift reaction of carbon dioxide.
Meanwhile, in a fuel processing system employing the PROX catalyst and the methanation catalyst, the operating temperature of the PROX catalyst is in the range of 100 to 200° C. and the operating temperature of the conventional methanation catalyst is in the range of 220 to 300° C. Thus, it is difficult to set an appropriate operating temperature range when the two catalysts are used together, and separate reactors are needed for the PROX reaction and the methanation reaction.